1. Related Field
The present invention relates to a low signature jet propulsion motor outlet.
2. Description of Related Art
Stealth technology is a sub-discipline of military tactics, which aims at designing e.g. aircrafts, ships and missiles which are difficult to detect with e.g. radar, infrared or sonar. This can be done by developing a particular shape for the surfaces that tends to reduce detection, by redirecting electromagnetic waves from radars. Another way is to cover the construction with a radar absorbent material that reduces or blocks radar signals. Shields that hide a heat source can be effective against infrared detectors.
When designing an aeroplane with a low signature the jet motor outlet causes problems. Motor outlets that are designed to have a low signature normally have a constant cross section area, i.e. the size of the outlet cannot be changed. This makes it very difficult to adapt them to velocities above the speed of sound or to use an after-burner. Some outlets have a rectangular cross section, which leads to a heavy and expensive construction that is not optimal in a stealth perspective.
Conventional aeroplanes normally use an outlet with a circular cross section since they have high performance and a low weight. However, from a signature point of view, a circular cross section is ineffective, since the curved outer surfaces give reflexes within wide angles. One solution to combining a low signature with a circular outlet is to hide the outlet with e.g. stabilizers or tail booms on both sides of the outlet. GB 2428414 A shows an aircraft which engines are mounted forward of the trailing edge of a wing and between two vertical stabilizers. The benefit with this location is to reduce ground detectable acoustic signatures and infra red signatures. The construction suffers from the drawback of aerodynamic problems like e.g. large surfaces and a large cross section area in the rear parts of the aeroplane and a squeezed flow.
The form and size of a circular motor outlet can be adjusted to different flight situations by providing controllable border surface elements at the periphery of the outlet. Those border surface elements can be hinged in the fixed channel of the motor outlet and be controlled by e.g. hydraulic means. A problem that arises with these variable circular outlets is the leakage of noise and heat from the edges and from the gap between the border surface elements.